1. Field of the Invention
This invention relates to a timing and synchronization by which a digital communication system, composed of many nodes interconnected by transmission paths characterized by various media, can be brought into synchronism. The signals on these transmission paths are asynchronous with respect to one another. The invention further relates to clock synchronization at system nodes especially but not exclusively to nodal subsystems that require a timing signal for operation, for example, time division multiplexers, modulation devices, security devices, or the nodal processors in communication switches.
2. Description of the Prior Art
For a multi-node digital communication, the maintainance of clock synchronism at all nodes is a formidable problem. The use of highly stable clocks, one at each system node, has been proposed as a solution to the problem, but this proposal appears to have certain deficiencies. The timing device would be expensive and the clock system would require periodic adjustment necessitating interruption of received transmission at a node caused by differences in the received transmission rates varying as a function of such physical transmission line characteristics as temperature, humidity and atmospheric disturbances.
Another proposed solution is a common timing signal transmitted from a single designated node to all other system nodes. This proposed solution appears to have certain deficiencies in that it would require an expensive timing signal transmission system to insure that the timing signal to all other nodes was not lost anywhere in the system. Furthermore, transmission path delay variations, as previously mentioned, would cause the nodes receiving the timing signals to respond to these variations and hence these clocks would exhibit timing transients which would result in abnormal nodal operation. Since timing of high bit rate digital streams is critical, a transient timing signal would make such time almost impossible.
A technique to achieve nodal synchrony is disclosed by J. S. Mayo in U.S. Pat. No. 3,136,861 issued on June 9, 1964. In accordance with that invention, each pulse signal to be multiplexed has its pulse repetition rate raised to a common repetition frequency by the insertion of control signals into the pulse signal. After multiplexing, transmission, demultiplexing and receiving the transmitted signals, predictive techniques are employed to remove the inserted control signals even in the presence of large transmission error rates. The predictive techniques determine when a control signal should have occurred in the transmitted signal, and when a control signal is lost due to transmission error. This determination is used to minimize the loss of information due to framing error. In addition, because of the composition of waveforms of higher frequency and the predictive techniques employed, there is no necessity for transmitting additional information regarding the composition of the signals of higher frequency. Because the control information is inserted into the original pulse signal at the originating node and transmitted to the receiving node, it must be received error free. If not, the necessary removal of the control information pulse may not occur, and though the control information is received correctly, it is used to delete pules from the data stream thereby creating gaps in the stream which must be smoothed to restore the pulse stream to its original form. This action must be accomplished before the pulse stream can be processed at the receiver node. For example, if the pulse stream were originated at a node and destined for a distant node and the path traversed several interim nodes, this addition and deletion of control information would be accomplished between every node path in the pulse stream path.
A disclosure in U.S. Pat. No. 3,042,751 issued to R. S. Graham on July 3, 1962 discloses a technique for achieving system synchrony that preceeded Mayo's disclosure. Although similar in structure, Graham's techniques used a variable delay insertion technique for control. However, unlike Mayo's disclosure, the delay was encoded and transmitted on a separate channel than that of the transmitted pulse signal being controlled. This technique reduces available channel capacity with respect to other pulse stream transmission. For this technique, the control information is subject to the same problems of error free transmission as with Mayo's, but in addition, the control channel must not fail. Otherwise, control information of the pulse stream would be completely lost.
Another technique to achieve network synchronization was disclosed by W. T. Deurdoth in U.S. Pat. No. 3,467,779 issued on Sept. 16, 1969. In this disclosure, a continuous correction derived from a pulse stream interim storage element under control of a master timing oscillator was used to increase or decrease the frequency of the master timing oscillator depending on the state of the storage element. Because of the continuous nature of the frequency correction applied to the master oscillator, any disruption of the pulse stream would cause immediate changes in the state of the storage element which would immediately cause the master oscillator to change frequency. But this would cause disruptive changes in the timing of other streams under the control of the master oscillator. This affect would propagate to other nodes through pulse streams timed by the master oscillator and a timing transient would result.